The present invention relates to catalysts for the full oxidation of volatile organic compounds (VOC), particularly hydrocarbons, and to a method for the full oxidation of volatile organic compounds (VOC) by using said catalysts.
The total combustion of VOC to CO2 and H2O becomes necessary in view of the toxicity and environmental impact of most unburnt VOC. The goal is to minimize both the release of VOC into the atmosphere and the formation of CO, which is also a toxic component.
The catalysts mostly used for VOC combustion are:
a) catalysts based on noble metals, which are characterized by a high cost but by excellent performance in terms of VOC conversion, said catalysts operating at temperatures between 200 and 450xc2x0 C. according to the reactivity of the compound;
b) catalysts based on mixed oxides, typically chromites of copper or of other metals, or barium hexaaluminate, which are characterized by a lower cost but are active in more drastic conditions (temperatures between 400 and 600xc2x0 C.). This second class of catalysts is also used for catalytic combustors for power generation units. In this case, they operate at temperatures above 900xc2x0 C.
Conventional catalysts used for the combustion of VOC are not free from drawbacks, i.e., high cost (for those based on noble metals) and poor activity (for the second class, accordingly requiring operation at higher temperatures, in conditions in which morphologic or structural transformations are facilitated).
A principal aim of the present invention is to eliminate the drawbacks of conventional catalysts for the oxidation of VOC, particularly catalysts for the full oxidation of VOC.
This aim and other objects which will become apparent hereinafter from the following detailed description of the invention are achieved by catalysts according to the present invention, which comprise one or more crystalline compounds having the formula
A2B3O6xc2x1d
where A is an alkaline-earth metal, an alkaline metal, a lanthanide, or a solid solution thereof, B is a transition metal, an element of group III, or a solid solution thereof, and d has a value between 0 and 1.
Advantageously, A is chosen from the group constituted by barium, cesium, potassium, strontium, and solid solutions thereof.
Conveniently, B is chosen from the group constituted by copper, nickel, manganese, iron, palladium, titanium, aluminum, gallium, zinc, cobalt, and solid solutions thereof.
Examples of catalysts according to the invention, wherein A is a solid solution of the above-mentioned cations, include compounds having the formula
(Ba2xe2x88x92xSrx)Cu3O6xc2x1d
with values of x up to 0.75.
Other examples of catalysts according to the invention, wherein B is a solid solution of the above-mentioned cations, have the formula
(Ba2Cu3xe2x88x92yPdy)O6xc2x1d
with y up to 0.33;
Ba2(Cu3xe2x88x92yNiy)O6xc2x1d
with y up to 1.0.
Methods for preparing the compounds included in the catalysts according to the present invention are disclosed in Italian patent application BO 96 A 000063. Said patent application describes the use of these compounds to fix gases, composite materials comprising said compounds, films made by said compounds, gas fixing devices comprising said compounds, and electrical and optical gas concentration sensors comprising said compounds.
The catalysts for catalytic combustion according to the present invention are characterized by a high activity, comparable with that provided by the more expensive noble metals, by a high resistance to temperature and to extreme operating conditions, by low cost and easy production even in the form of compounds and thin films. Moreover, differently from any material hitherto known, they are totally selective for carbon dioxide, CO2, with respect to carbon monoxide, CO. This means that in any operating condition for the catalysts according to the present invention the only carbon oxide produced by the reaction is CO2.
The present invention also relates to methods for the full oxidation of VOC which use the catalysts according to the present invention.
The methods for the full oxidation of VOC according to the present invention relate to the full oxidation of VOC in combustion chambers or in afterburning chambers or in combustion-gas chambers.
The oxidation reaction occurs in a fixed bed or in a fluidized bed.
The catalysts according to the present invention lead to full oxidation of VOC. In this manner, the gas mixture produced by the oxidation of the VOC does not contain carbon monoxide but contains only carbon dioxide. In this manner, the operations for eliminating carbon monoxide, and the known negative consequences of its presence in the environment, are avoided. Moreover, by oxidizing the carbon monoxide to carbon dioxide, the full potential of the VOC Oxidation reaction is utilized, with obvious advantages in terms of energy.
The Applicants have found that the total VOC oxidation reaction in the presence of the catalysts according to the invention occurs with a high conversion of VOC even at low temperatures.
Moreover, the Applicants have found that the catalysts according to the present invention allow full oxidation of VOC even in conditions entailing a significant oxygen deficit.
Besides, it has been found that the catalytic activity of the catalysts according to the present invention is not negatively affected by prior heating under reaction up to high temperatures (for example up to 920xc2x0 C.) and cooling to ambient temperature.
Moreover, analysis of reaction selectivity data suggests that the catalysts according to the present invention are capable of directly converting CO to CO2 at temperatures above 250xc2x0 C. and in the presence of even minimal oxygen concentrations.